Musnad Ahmad 926:7

Abdullah narrated to us, Ja’far ibn Mihran al-Sabbak al-Basri narrated to us, Abd al-Warith narrated to us, from Muhammad ibn Jahadah, from al-Hakam, from Mujahid, from Abd al-Rahman ibn Abi Layla, from Ubayy ibn Ka’b: That Gabriel came to the Prophet, peace and blessings of Allah be upon him, while he was at the watering place of Banu Ghifar. He said: "Indeed, Allah commands you to have your nation recite the Quran according to one mode (harf)." He said: "I ask Allah for His pardon and forgiveness." Then he mentioned the hadith, until he said: "Indeed, Allah commands you to have your nation recite the Quran according to seven modes. So whoever recites according to any one of them, he is as he said."